Huaiyu H. Chen-Mayer

High-resolution charged particle and neutron imaging using charge injection devices

A charge injection device (CID) camera and image processing system have been used as a position sensitive detector for energetic charged particles and low energy neutrons. This video radiation detector (VRD) is simple in design but highly effective for real-time radiography and dosimetry with many advantages characteristics. The VRD currently has a dynamic range of 65,000 intensity levels for a 755 X 484 pixel matrix, an active area of 7 mm X 9 mm, a spatial mapping resolution of about 14 micrometers for single detected events (7 micrometers for radiation from a point source), and is sufficiently radiation-hard to be operated in a neutron beam for extended periods of time. Radiation images are updated at a rate of thirty frames per second. The VRD is sensitive to fission fragments, alpha particles, and slow neutrons. Using commercially available image processing hardware and software and an off-the-shelf camera, the system is inexpensive, easy to use with simple interpretation of data, and is capable of performing radiography with only minimal adaptations. Applications in our laboratory include the characterization of focused cold neutron beams, the mapping of uranium and lithium distributions in samples by the detection of neutron absorption reaction products, and the mapping of spontaneous alpha radioactivity from environmental samples. Results provide information on x-y position, counts received, and energy deposited per count, each as a function of time.

Characterization of a cold neutron beam from a curved guide

Abstract A supermirror guide that includes both straight sections and curved channels has been installed at a cold neutron source facility. A description of the beam line and the beam characteristics are given. The transmitted neutron wavelength spectrum and the beam current density have been measured. Factors that affect the spectrum are discussed. Though the final straight guide section improves the spatial uniformity of the beam intensity at the guide exit, the spatial-angular correlations caused by the curved channels persist. These correlations are observed in the beam divergence measurements at various positions along the beam line using neutron imaging plate technology, and are explained by ray projections. The divergence of the transmitted beam is determined both by the critical angle of the guide and the collimation beyond the guide.

A polycapillary bending and focusing lens for neutrons

A glass polycapillary lens that both bends and focuses a cold neutron beam has been designed and constructed. The bender focuser guides part of the incident beam away from its line of sight and focuses it to a spot of width 0.65 mm at a distance 95 mm from the lens exit and 20 mm below the bottom edge of the beam path, with a gain of 20 in neutron current density. The neutron transmission characteristics of the lens have been determined with two types of position-sensitive detectors, a charge injection device, and an imaging plate. The lens has been tested with prompt gamma measurements on a gadolinium shard and titanium foil.

Capillary neutron optics for prompt-gamma activation analysis

A neutron lens has been constructed to focus cold neutrons from the exit of a58Ni neutron guide, which delivers a beam to the Prompt-Gamma Activation Analysis (PGAA) station at the NIST Cold Neutron Research Facility. The lens compresses a neutron beam of cross section 50 mm× 45 mm onto a focal spot of diameter 0.53 mm (fwhm) wich an average gain of 80 in neutron current density. PGAA measurements have been performed to demonstrate the enhanced sensitivity and detection limits for various elements and the spatial resolution in one transverse dimension. For the two test particles (a gadolinium glass bead and cadmium metal of sizes less than 0.5 mm), the gain in the γ-count rate with the lens is a factor of 60, and the detection limit is improved by a factor of 20. The system can be used for two-dimensional mapping of samples on a sub-millimeter scale to complement other analytical techniques such as neutron depth profiling (NDP).

Standardizing CT lung density measure across scanner manufacturers

Purpose: Computed Tomography (CT) imaging of the lung, reported in Hounsfield Units (HU), can be parameterized as a quantitative image biomarker for the diagnosis and monitoring of lung density changes due to emphysema, a type of chronic obstructive pulmonary disease (COPD). CT lung density metrics are global measurements based on lung CT number histograms, and are typically a quantity specifying either the percentage of voxels with CT numbers below a threshold, or a single CT number below which a fixed relative lung volume, nth percentile, falls. To reduce variability in the density metrics specified by CT attenuation, the Quantitative Imaging Biomarkers Alliance (QIBA) Lung Density Committee has organized efforts to conduct phantom studies in a variety of scanner models to establish a baseline for assessing the variations in patient studies that can be attributed to scanner calibration and measurement uncertainty. Methods: Data were obtained from a phantom study on CT scanners from four manufacturers with several protocols at various tube potential voltage (kVp) and exposure settings. Free from biological variation, these phantom studies provide an assessment of the accuracy and precision of the density metrics across platforms solely due to machine calibration and uncertainty of the reference materials. The phantom used in this study has three foam density references in the lung density region, which, after calibration against a suite of Standard Reference Materials (SRM) foams with certified physical density, establishes a HU‐electron density relationship for each machine‐protocol. We devised a 5‐step calibration procedure combined with a simplified physical model that enabled the standardization of the CT numbers reported across a total of 22 scanner‐protocol settings to a single energy (chosen at 80 keV). A standard deviation was calculated for overall CT numbers for each density, as well as by scanner and other variables, as a measure of the variability, before and after the standardization. In addition, a linear mixed‐effects model was used to assess the heterogeneity across scanners, and the 95% confidence interval of the mean CT number was evaluated before and after the standardization. Results: We show that after applying the standardization procedures to the phantom data, the instrumental reproducibility of the CT density measurement of the reference foams improved by more than 65%, as measured by the standard deviation of the overall mean CT number. Using the lung foam that did not participate in the calibration as a test case, a mixed effects model analysis shows that the 95% confidence intervals are [−862.0 HU, −851.3 HU] before standardization, and [‐859.0 HU, −853.7 HU] after standardization to 80 keV. This is in general agreement with the expected CT number value at 80 keV of −855.9 HU with 95% CI of [−857.4 HU, −854.5 HU] based on the calibration and the uncertainty in the SRM certified density. Conclusions: This study provides a quantitative assessment of the variations expected in CT lung density measures attributed to non‐biological sources such as scanner calibration and scanner x‐ray spectrum and filtration. By removing scanner‐protocol dependence from the measured CT numbers, higher accuracy and reproducibility of quantitative CT measures were attainable. The standardization procedures developed in study may be explored for possible application in CT lung density clinical data.

A monolithic polycapillary focusing optic for polychromatic neutron diffraction applications

We have conducted measurements at five different thermal neutron wavelengths to determine the transmission characteristics of a tapered monolithic focusing lens with a focal length of 100 mm, suitable for time-of-flight diffraction. Both the width of the focused beam and the intensity gain of the optic increase as a function of wavelength. We have performed similar measurements on a polychromatic beam on a pulsed neutron source, where the results are subject to background from short wavelength neutrons. The use of a beryllium filter shows the increased effective gain for the longer wavelengths at the expense of an increased focused beam width by a factor of 2.

Use of neutron beams for chemical analysis at NIST

At the National Institute of Standards and Technology, there are two techniques for chemical analysis that use neutron beams from the reactor for target irradiation: neutron depth profiling (NDP) and prompt γ-ray activation analysis (PGAA). There are two facilities for each technique, one equipped with a thermal neutron beam and the other, with a cold neutron beam. In addition, focused beams of cold neutrons will be used to measure the two-dimensional element distributions by PGAA and three-dimensional distributions by NDP. This paper includes a brief description of the facilities, the measurement capabilities of each, some recent applications of NDP and PGAA, and neutron focusing as applied to these techniques.

In situ measurement of lithium movement in thin film electrochromic coatings using cold neutron depth profiling

Direct analysis of lithium movement within multilayer thin-film electrochromic (EC) coatings has been performed by cold neutron depth profiling (CNDP). Transfer of lithium between a counter-electrode layer and an EC tungsten trioxide layer controls the optical density of the EC coating. The lithium profiles have been measured using the CNDP instrument at the Center for Neutron Research at the National Institute of Standards and Technology. The lithium depth profiles are based on measurement of the energy of alpha particles from the 6 Li(n,α)3H reaction. The alpha particles lose energy as they exit the film and this energy loss provides direct measurement of the depth of the originating lithium nucleus. In this case, in situ measurements were taken with different bias voltages on the film layers. The bias causes the lithium to migrate between different layers, changing the optical density of the films. A great advantage of the CNDP technique is that it is non-destructive, which allows repeated observation of the lithium movement. Results of simultaneous optical transmission measurements and lithium profile measurements are reported. Copyright

Coincidence and anti-coincidence measurements in prompt gamma neutron activation analysis with pulsed cold neutron beams

A novel approach is implemented to alleviate some persistent problems in neutron capture prompt gamma activation analysis (PGAA). Detection sensitivities of PGAA are often restricted by the following factors: poor signal to noise ratios, interferences from background signals, and, in some cases, overlapping energy lines from different origins, namely ultra short-lived decay lines interfering with prompt decay. Timing the gamma-ray acquisition with the actual capture events using a pulsed beam of cold neutrons allows discrimination between prompt and delayed emissions from a sample source as well as against background events. Coincidence gating selects the prompt gamma-ray emissions. Contributions of background capture gamma-rays are suppressed because of different flight times of neutrons to the sources of background radiation, providing a reduction in direct gamma-ray interferences. Anti-coincidence gating allows measurement of only decay radiation that originates from short-lived activated states of the nuclides after capture. Spectra of decaying nuclides are free of interfering prompt activities, as well as have lower continuum background from Compton scattering of high-energy prompt gamma-rays in the detector. The measurements provide the opportunity to use ultra-short half-life nuclides for analytical purposes, no sample transfer times are lost, and repetitive activation and counting cycles are achieved with the use of pulsed neutron beams.

Cold neutron incoherent scattering for hydrogen detection in industrial materials

Neutron incoherent scattering (NIS) is proposed as a method for the rapid detection of hydrogen and corrosion in industrial materials. As an example, we apply the NIS method to the detection of hydrogen in graphite and in titanium alloy, and compare the results with those obtained with the prompt gamma activation analysis (PGAA) method. The scattering cross section for hydrogen is much greater than the capture cross section, which enables the NIS method to have a detection limit and accuracy close to that for PGAA, and allows real time experiments on hydrogen detection to be performed. We report preliminary results on using the NIS method to determine hydrogen in urea [CO(NH2)2]/graphite and in titanium matrices.